Alveolar epithelial cells (AEC) cover the alveolar surface of the lung, which is the major acid-excreting organ in mammals. Maintenance of intracellular pH (pHi) is a critical factor for intracellular homeostasis in AEC. pHi and the acid/base transport proteins that modulate pHi may also be important mechanisms for signal transduction and regulation of diverse processes, including cell growth and differentiation. The candidate have established that alveolar epithelial cell pHi is regulated by several ion transport mechanisms that are capable of effecting acid/base entry or extrusion from the cell, and have also shown that AEC express the NHE1 isoform of the mammalian Na/H exchanger (NHE) on their basolateral cell membranes. The objectives of the studies proposed herein are to further characterize acid/base transport and homeostasis in alveolar epithelial cells, and to study the relationship between NHE1 expression and activity and alveolar epithelial cell biology. The candidate will use monolayers of rat alveolar epithelial cells grown in primary culture on tissue-culture treated polycarbonate filters for many of these studies. This model is especially well suited for studies of AEC acid/base transport, epithelial cell polarity, and growth and differentiation. Where possible, the candidate will validate his results using in situ studies on rat alveolar epithelium. Based on the experimental results of his ongoing studies of alveolar epithelial acid/base transport and intracellular pH regulation, he became interested in testing the hypotheses that (1) NHE1 contributes to intracellular homeostasis in AEC by functioning as a mechanism of signal transduction; (2) NHE1 is regulated in AEC in a cell-specific fashion; (3) Regulation of NHE1 is important for alveolar epithelial cell growth and differentiation. In order to test these hypothesis, the candidate will investigate the following Specific Aims: (#l) To study the relationship between basolateral expression of NHE1 and the effects of the extracellular milieu on intracellular pH; (#2) To determine the role of NHE1 in homeostatic responses to changes in cell volume and osmolarity; and (#3) To investigate the relationship among NHE1 activity, intracellular pH, and alveolar epithelial cell differentiation. The long-term goals of this project are to define the role of NHE as a mechanism of signal transduction and cell regulation in AECs, inclusive of and in addition to its nominal role in maintenance of alveolar epithelial cell pH.